Removal of acidic components from smelters, power plants, refineries, and petrochemical plants has been a pervasive problem for decades.
While a variety of wet scrubbing techniques have been tried to remove acidic sulfur and halide compounds from vapors and liquids, such processes are difficult. Wet scrubbing is effective, but requires that the gas being treated be cool enough to support a liquid phase, and the flue gas from such a process makes a very visible plume when discharged to the atmosphere. Although the white plume is simply condensed water vapor, it is very visible and somewhat objectionable to some members of the community.
Many process streams also do not lend themselves to wet scrubbing processes, in that the streams are parts of processes which require a dry atmosphere. Wet scrubbing removes, e.g., chlorides, but can add so much water that other parts of the process would be troubled. Leaving the acidic components in a process stream unfortunately also causes problems, as can be seen from a review of the problems of chlorides in reformate which follows below.
Catalytic reforming, using Pt based reforming catalyst, is one of the most important refinery processes in the world. Most refineries have a catalytic reformer, which converts naphtha fractions into high octane reformate.
Reformers come in many types and sizes--from 2000 BPD fixed bed units to moving or swing bed units processing more than 50,000 BPD. Reformers are available with fixed bed reactors, swing bed reactors, or moving bed reactors. Many new units are moving bed reactors, available from UOP, Inc, Des Plaines, Ill.
Reformers generally use mono-metallic catalysts (Pt on a support such as alumina) or bi-metallic catalyst (Pt--Re on a support). Other combinations of Pt and other metals are known.
All reforming catalysts are believed to contain halide, almost invariably chlorine. Chlorine is now ubiquitous in catalytic reforming. Chloroplatinic acid may be used in the impregnation solution forming the catalyst. Some refiners may add chlorine compounds during normal operation.
One major oil company developed a Pt reforming catalyst regeneration or "rejuvenation" procedure which conducted at least some portions of the regeneration in the presence of one or more chlorine compounds. The procedure was believed originally developed for swing reactor systems which were regenerated every day or so, but this regeneration method, or some variant of it, was eventually used in semi-regenerative reformers and in moving bed reformers.
All of this chlorine can, and does, find its way into gas and liquid products from the reformer. Based on a review of several decades of The Oil and Gas Journal, the key to successful catalytic reforming is lots of chloride. For decades refiners have talked about the problems of getting enough chlorides into the system, and dealing with the chlorides in the vapor and liquid products from the reformer.
In 1977 there was talk of the need for heat, chloride and moisture to redistribute platinum.
In 1980 there was a discussion of deposits of ammonium chloride in catalytic reforming compressor internals.
In 1985 there was discussion of the need for, and difficulty of maintaining, 1.0 wt % chloride on bimetallic catalyst between regenerations. It was suggested to "come out on the high side on chloride. "
In Alumina adsorbents effectively remove HC.sub.1 from reformer H.sub.2 gas stream, Janke et al, Oil and Gas Journal, May 12, 1986, page 64, talked about controlled injection of organic chloride at the reformer reactor inlet, and the mischief caused by all this chloride. The problem was worse with continuous catalytic reforming processes, which were reported "to require higher levels of chloride addition for regeneration . . . ." The solution proposed was use of alumina adsorbents to remove the HC.sub.1 from the net off gas. This article is incorporated by reference.
In Apr. 1, 1994 there was a discussion of corrosion in fired heaters due to chloride in the hydrogen from the reforming unit. The proposed solution was alumina treaters.
The problem is not limited to reformers. Similar problems occur in some isomerization units, and may occur in other units which are relatively dry and use a chloride containing catalyst.
The conditions which lead to chloride problems are catalysts which contain, or reaction conditions which require, chlorine compounds, and reactants which are dry enough that no separate aqueous phase forms in the vapor/liquid separator downstream of the reactor. Essentially all Pt reformers meet these conditions, and many isomerization and other processing units meet these conditions.
The situation could be summarized as follows for Pt reformers. Although refiners may use different reforming catalysts, all the catalysts seem to contain chlorine. There is enough chlorine either present in the virgin catalyst, or from chlorine addition during reformer operation, or from chlorine added during the catalyst regeneration, so that chlorine compounds appear in all the product streams coming from the reformer. Both vapor and liquid products have chlorine compounds.
The raw liquid reformate has chlorides. The net hydrogen gas make has chlorine compounds. When the raw reformate is fractionated, usually in a debutanizer, the overhead vapor fraction contains chlorine compounds.
While chlorides in liquid reformate are a serious problem, the present invention is not directed to solving that problem. Instead, the present invention focusses on removal of chlorides or other acidic halides present in dry gas streams such as gas streams from a reformer. Of primary concern is removal of chlorides from the net gas make from the reformer vapor liquid separator, the hydrogen rich gas removed from the reformer for use in other refinery processes, and the flue gas from any catalyst regeneration/chlorination facility which may be present.
In reforming units with recontacting drums for recycle gas, it would help if some means were available to remove chlorides from recycle gas. There is an equilibrium between chlorides in reformate and chlorides in the gas phase, and removing chlorides from recycle gas would reduce the amount of chlorides in the liquid reformate as well as reduce chlorides in the net recycle gas make of the unit.
Another concern is removal of chlorides from vapor streams generated by downstream processing of raw reformate, e.g., removing chlorides from overhead separator vapor associated with reformate fractionators.
It would be beneficial if a process were available which could remove chlorides and the like from flowing process streams in a completely dry process, not involving the presence of any aqueous phase.
Those skilled in the treating arts know that some reaction of acidic halides in liquid hydrocarbons can occur with solid caustic, always accompanied by formation of salt, and usually in the presence of significant amounts of dissolved water.
A bed of granular alkalies was used to treat a variety of liquid hydrocarbon streams in Sun U.S. Pat. No. 3,761,534, which is incorporated by reference.
Example 1 used 4-8 mesh granular NaOH to remove sulfuric acid from an alkylate stream of tert.-butylated ethyl-benzene containing about 0.3N total acid, primarily sulfuric acid. Although efficient acid removal first occurred, the bed plugged before 100 volumes of alkylate could flow through the bed.
Example 4 used no NaOH, but treated an effluent from the alkylation of benzene with ethylene in the presence of HCl with soda lime and glassmaker's (G.M.) alkali to remove acid.
Example 5 used pellets of C. P. NaOH to treat crude tert. butylated ethyl-benzene containing 570 ppm H.sub.2 SO.sub.4. NaOH pellets plugged at 92 weights of alkylate per weight of alkali, while beds of soda lime and G. M. alkali did not plug.
Example 7 used G. M. alkali on a support grid to treat crude tert.butylated ethylbenzene containing about 600 ppm sulfuric acid. The organic flowed up through the support grid, through the alkali to an outlet above the bed of alkali. A white precipitate built up in the reservoir below the grid, which was periodically removed through a drain valve by a water purge. The bed of alkali was reported essentially unchanged by casual observation and there was no increase in resistance to flow through it.
The streams treated in '534 were probably saturated with water, as periodic water purges were reported in many examples, and salts seemed to collect as solid deposits in a sump under the bed of alkaline solid. Some of the results reported could be summarized as follows:
Beds of caustic pellets do not work for very long to remove acidic contaminants from liquid hydrocarbon streams.
All beds plug in downflow operation or rapidly lost effectiveness. Upflow operation with alkali on a support of a grid or coarse screen works a long time because salts that form can fall down through the screen.
Porous G.M. alkali was better than solid caustic.
There is a need for a process that can treat bone dry gas streams at a temperature so high that no aqueous phase could be maintained. If such streams could be treated with solid caustic in a way which produced very little in the way of waste streams or byproducts, it would be a great advance in the art.
I discovered a way to treat such gas streams. Chlorides in hot, bone dry gas could be efficiently converted to salt. I also developed a solid caustic regeneration procedure, which allowed the salt to be removed as a dry powder from the solid caustic particles, permitting reuse of the remaining caustic and recovery of a low volume, almost neutral salt as the only product of the neutralization reaction. This regeneration procedure could be used to continuously regenerate caustic used in a hot, dry gas treating process. This regeneration technique could also be used to regenerate salt contaminated particles of solid caustic from other sources, so long as the salt had been deposited on the solid caustic in a dry atmosphere.